Carbon-filled materials have been widely used to form electrically-conductive members of various electrical equipment and components, including high voltage armature (stator) bars of generators. As known in the art, armature bars are composed of a number of conducting copper strands that are insulated from each other by strand insulation. The strands are typically arranged to form two tiers or columns that are separated by an insulating strand separator, which together may be termed the "bare bar." Surrounding the tiers is a groundwall insulation typically formed by multiple wrappings of a mica paper tape. As used herein, "high voltage" refers to an armature or stator bar subjected to voltages of 6 kV and higher.
As many as three different conductive materials are typically present in a high voltage armature bar. On the outside surface of the groundwall insulation in the slot section, a coating of conductive paint or a conductive tape is used as conductive slot armoring to protect the groundwall insulation from erosion by electrical discharges. Another method of forming protective conductive slot armoring is to apply a conductive adhesive on the groundwall insulation, wrap a woven fabric tape over the adhesive to bond the tape to the groundwall insulation, and then coat the tape with a conductive paint. The surface resistance of the slot armoring is usually desired to be about 500 to 100,000 ohms/square in order to suitably protect the groundwall insulation from electrical discharges. For this purpose, the conductive paints and tapes of slot armorings have contained carbon dispersed as a filler in a polymeric or resinous binder. However, after long term use and exposure to high voltage and thermal stresses, sections of the tape and paint can deteriorate. This is visible as white spots due to the decomposition and volatilization of the carbon and binder as gaseous decomposition products.
Another conductive material used within high voltage armature bars is referred to as conductive internal grading, which may be in the form of a coating of paint or a tape applied over the bare bar prior to application of the groundwall insulation. Internal grading paints are usually applied to the top and bottom edges and corners of the bare bar, extending about three to twenty-five millimeters on the sides of the bar. Internal grading tapes are typically a porous fabric support sheet that carries a conductive polymeric coating of carbon particles dispersed in a polymeric matrix, as described in U.S. Pat. No. 5,723,920, assigned to the assignee of the present invention. Internal grading is used to achieve an equipotential voltage plane, eliminating voltage stress risers that cause reduced dissipation factor tip-ups, which is the difference in dissipation factor at different stress levels. Internal grading can also improve the short time breakdown strength of the insulation, as discussed in the U. S. Pat. No. 5,723,920. However, carbon-based internal grading paints and tapes are susceptible to deterioration around the high stress corners of the bar and on the top and bottom edges of the bar. The deterioration is visible by the absence of the carbon after long term exposure to high voltages under accelerated test conditions, in which applied voltages are much higher than normal operating voltages.
The third type of conductive material sometimes present in high voltage armature bars is referred to as a transposition filler, which is typically formed by a compound that is molded on the top and bottom edges of the bare bar. The molding compound is usually a catalyzed epoxy resin that fills the voids between the copper strands at the crossovers, providing smooth surfaces at the top and bottom of the bar after molding. As an alternative to conductive internal grading tapes and paints, the molding compound can be made conductive by dispersing carbon particles in the resin. Conductive molding compounds of the type used for transposition fillers are also sometimes used as a component of the internal grading system.
As indicated above, in the past conductive paints, adhesives, tapes and molding compounds used in high voltage armature bars have typically contained carbon as the conductive filler, which is dispersed in a polymeric matrix. The carbon is present as a conductive carbon powder or powdered graphite, which renders the material black. The conductive paints, adhesives and tapes used as conductive slot armoring for groundwall insulation and the conductive paints and tapes used for internal grading at the interface between the groundwall insulation and the bare bar have given excellent performance. Conductive 15 molding compounds on the top and bottom edges of the bare bar have also performed well. Nonetheless, further improvements in the manufacturability and performance of these conductive materials would be desirable. For example, the manufacture of conductive paints, adhesives and tapes containing carbon as a filler is made difficult because of the large changes in surface resistivity with small changes in carbon content. As indicated above, the surface electrical resistance of conductive paints and tapes used to form the protective slot armorings and internal grading must be within a certain range, usually from about 500 to 100,000 ohms/square. When carbon is used as the filler to impart conductivity, the surface resistance can fall below or rise above the desired range as a result of a small excess or deficiency of the carbon content, respectively. Changes in processing conditions can also significantly alter the surface resistance of these materials. Consequently, caution is required in formulating and manufacturing carbon containing conductive materials for armature bars because of the strong sensitivity of the surface resistivity to the carbon content.
In addition it is believed that, under high voltage conditions, the carbon particles decompose by oxidation to carbon monoxide and carbon dioxide, causing the black color of the carbon to disappear and producing white areas in the material. It is possible that under high voltages, oxygen in the air forms ozone that acts as an oxidizer of the carbon. Decomposition of carbon in the slot armoring, internal grading and transposition filler of an armature bar lowers their conductivity and therefore negatively affects their performance.
In view of the above, it would be desirable if improved paints, adhesives and tapes were available for use as conductive materials for high voltage armature bars, and particularly materials that exhibit improved resistance to erosion by electrical discharges, improved resistance to oxidative decomposition, and less sensitivity to surface resistance.